Chemical vapor deposition (CVD) of epitaxial layers on semiconductor substrates is well known in the semiconductor manufacturing art. Various materials, including silicon carbide (SiC), may be employed as epitaxial layers. Specifically, the chemical vapor deposition of SiC presents a number of problems to those in the semiconductor manufacturing industry.
Silicon carbide has long been a candidate material for use in electronic devices, particularly those intended for high temperature, high power, and high frequency use. Silicon carbide has an extremely high melting point, a relatively large energy bandgap, a high saturated electron drift velocity, a high breakdown field strength, a high thermal conductivity, and superior chemical resistance. Its large energy bandgap also makes it an excellent material for blue light emitting diodes (LEDs) and for use in radiation intensive environments.
In particular, intrinsic silicon carbide epitaxial layers grown by chemical vapor deposition have a carrier concentration of nitrogen that generally is always at least 1.times.10.sup.17 atoms per cubic centimeter (cm.sup.-3 ("1E17"). Conventional wisdom holds that this intrinsic nitrogen is a consequence of nitrogen present in the source and carrier gases used during chemical vapor deposition. The result is that intrinsic silicon carbide epitaxial layers will always have donor atoms present to at least this extent. Although this may not present a problem when the resulting silicon carbide is to be n-type, it raises significant difficulties when p-type silicon carbide is desired. Because of intrinsic nitrogen, such p-type epitaxial layers will always be "compensated" with a nitrogen donor to at least the extent of 1.times.10.sup.17 cm.sup.-3 (1E17). As is known to those familiar with semiconductor devices and technology, such compensation can limit the usefulness or application of devices produced using such materials.
For example, in rectifying diodes, a higher population of donor (n-type) carriers in the p-type material lowers the reverse breakdown voltage of the resulting rectifier. Stated differently, the development of rectifying diodes with desirable or necessary higher breakdown voltages requires a minimization of donor carriers in p-type layers. In light emitting diodes ("LED's), the presence of impurities such as intrinsic nitrogen brings a corresponding presence of undesired potential energy levels and unwanted transitions within the preferred bandgap. These undesired transitions generate photons of undesired wavelengths and thus pollute the color of light produced by the LED.
In conventional semiconductor techniques, however, the reasons for the presence of nitrogen as an unwanted intrinsic donor have not been recognized. This lack of recognition of the problem raised by nitrogen with respect to silicon carbide probably results from the fact that nitrogen does not act as a donor in silicon, the most widely used semiconductor material. Thus, the presence of 1E17 of nitrogen in silicon does not present the problem that such a concentration presents in silicon carbide.
Furthermore, chemical vapor deposition growth of silicon carbide typically takes place at temperatures much higher than the temperatures at which corresponding CVD growth of silicon takes place. For example, CVD of silicon takes place at temperatures of no more than about 1200.degree. C., while that of silicon carbide preferably takes place at least about 1500.degree. C. or higher. At higher temperatures, there exists a greater probability that the problem of impurities will be exacerbated. In CVD growth of silicon carbide, however, higher temperatures promote fewer defects in the resulting crystals. Thus, the higher quality LED's are preferably formed from epitaxial layers grown at least 1500.degree., and desirably at even higher temperatures, but conventional CVD systems and susceptors are only suitable at temperatures of about 1250.degree. C. or less.
In contrast to the conventional wisdom as to the root of the nitrogen contamination problem, the present inventors have discovered that much, and very likely all, of the nitrogen contamination in intrinsic silicon carbide is a result of nitrogen gas that escapes from the susceptors during chemical vapor deposition. Because CVD growth of silicon carbide typically takes place at temperatures well above those necessary for CVD growth of silicon, such "out gases" may not be generated during silicon growth, and the problems they raise have accordingly remained unobserved prior to the more recent advances in silicon carbide technology described earlier.
It has thus only recently been observed that at the high temperatures required for CVD growth of silicon carbide, the silicon carbide coatings on most graphite susceptors begin to develop minute mechanical failures, often exhibited as cracks or pinholes. Because graphite is an excellent absorbent for many gases, it appears that in some cases, gases trapped in the graphite before the susceptor was formed escape through these cracks and pinholes and contaminate the silicon carbide epitaxial layers grown using such susceptors. For example, in most susceptor manufacturing processes, the coating system is merely purged with hydrogen prior to being coated with silicon carbide, a step that takes place after the graphite has been exposed to nitrogen In some cases, nitrogen is intentionally added during the coating process. In other cases it appears that susceptors will absorb atmospheric nitrogen every time a CVD growth cycle is completed and the growth chamber opened.
Thus, the relatively high intrinsic donor concentration of epitaxial layers of silicon carbide grown by chemical vapor deposition has remained a problem for which conventional wisdom and techniques have failed to provide a solution.
In addition to the nitrogen contamination problem, other problems with the CVD process have presented themselves. One of these problems includes heating the semiconductor substrate during the CVD process. Many types of heating used in conventional CVD systems, such as radiant heating heat not only the substrate but also parts of the CVD system itself. As a result of such undirected heating, more time is required to heat the substrate. In addition, the undirected nature of radiant heating results in uneven heating throughout the substrate with accompanying defects in the resulting crystal structure of the epitaxial layer being formed.
These problems have recently been addressed in the aforementioned copending U.S. patent application Ser. No. 07/558,196, in which epitaxial layers of SiC are formed by reducing the carrier concentration of nitrogen. The nitrogen concentration is reduced by using a susceptor formed of a material that will not generate undesired nitrogen containing out gases at the temperatures at which CVD of SiC take place.
Another attempt at addressing the purity problem--although not for CVD growth of silicon carbide--has been to eliminate the susceptor in a vapor levitation epitaxy technique described by H. M. Cox, S. G. Hummel, and V. G. Keramidas at Bell Communications Research, Murray Hill, N.J. Their publications include "Vapor Levitation Epitaxial Growth of InGaAsP Alloys Using Trichloride Sources", Inst. Phys. Conf. Ser. No. 79: Chapter 13, page 735 (1986); and "Vapor Levitation Epitaxy: system Design and Performance", J. Crystal Growth 79 (1986) 900-908. These works never addressed, however, the specific problem presented by the growth of silicon carbide, which crystallizes in over 150 polytypes, most separated by very small thermodynamic differences.
Others have avoided use of a susceptor, for reasons other than reduction of nitrogen generation, by levitating the substrate through the use of source or other gases. For example, the Mammel U.S. Pat. No. 3,627,590 patent describes a CVD system for epitaxial growth of thin films on a substrate that is levitated using a gas flow. Mammel, however, uses radiant heating and susceptor heating and employs a face up orientation of the substrate during CVD.
Accordingly, it is an object of the present invention to provide an improved method and associated apparatus for producing epitaxial layers of such silicon carbide by chemical vapor deposition, to produce intrinsic silicon carbide in which the carrier concentration of nitrogen is less than 5.times.10.sup.16 cm.sup.-3, and to provide for heating of the semiconductor substrate in a CVD system which is directed, controllable and even.